Try this:
t = 1:48; % Time Vector
load = randi(1000, 1, 48); % Create Data
level_1 = 750;
level_2 = 600;
above_level_1 = load >= level_1; % Logical Index Vector
area_1 = trapz(t(above_level_1), load(above_level_1)); % Area Under ‘Level_1’
below_level_2 = load <= level_2; % Logical Index Vector
area_2 = trapz(t(below_level_2), load(below_level_2)); % Area Under ‘Level_2’
figure
plot(t, load)
hold on
plot(xlim, [1 1]*level_1)
plot(xlim, [1 1]*level_2)
hold off
grid
